Vapor compression systems (VCS) move thermal energy between a low temperature environment and a high temperature environment in order to perform cooling or heating operations in order to improve comfort of occupants in the environment. For example, heat can be moved from an indoor space to an outdoor space in order to lower the indoor temperature in a cooling operation, or heat can be moved from an outdoor space to an indoor space in order to raise the indoor temperature in a heating operation.
The heat load, or rate at which the thermal energy is moved into a space (e.g., by hot air passing into a building) is generally not directly measured, but its effect is detected as changes in the indoor space temperature or zone temperature. In order to control the zone temperature, the operations of the VCS modulates the cooling or heating capacity provided by the system to counteract the load such that the zone temperature is near a desired zone temperature. The thermal capacity of a heat exchanger is the rate at which the thermal energy is accepted or rejected by a heat exchanger.
A multi-zone vapor compression system (MZ-VCS) includes at least a single compressor connected to multiple heat exchangers arranged in one or more indoor zones. Conventionally, the heating or cooling capacity of such indoor heat exchangers is modulated by duty cycling each heat exchanger between “ON” and “OFF” modes of the operation. The heat exchanger is OFF when an inlet valve that controls refrigerant flow is closed or alternatively, the compressor that pumps refrigerant through the system is stopped, so that no cooling or heating is performed by the heat exchanger. The heat exchanger is ON when an inlet valve is opened and the compressor is operating so that the heat exchangers in the indoor zones operate at their full thermal capacity. A controller decides how to alternate between the modes based on a difference between the zone temperature and the desired zone temperature.
However, the act of switching heat exchangers ON and OFF, especially in MZ-VCS where the zone heat exchangers can be switched ON and OFF independently from each other, result in persistent variations in the outputs of the system, such as zone temperatures and heat exchanger temperatures, that are known to be energetically inefficient and reduce occupant comfort. Accordingly, there is a need in the art for a control system and method to smoothly control the thermal capacity of heat exchangers, such as the heat exchangers of MZ-VCS.
In addition to smoothly controlling the heat capacity within a heat exchanger in a multi-zone system, there is a need to control the overall operations of the multi-zone vapor compression system. For example, during the operation of the VCS, various constraints should be enforced. For example, certain maximum or minimum temperatures and pressures should not be violated for equipment safety. Some controllers enforce the constraints reactively, i.e., corrective action is taken once a dangerous situation is detected. The violations of the constraints can occur for some period of time while the system responds with corrective actions, and therefore the threshold at which corrective action is used is selected conservatively to account for violations that can occur. As a result, the controllers with reactive constraint management logic are often detuned away from the value of the constraints, which sacrifice the regions of highest performance, see, e.g., EP2469201.
Accordingly, there is a need in the art for a system and a method for an efficient control of MZ-VCS subject to constraints.